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Circuit and electromagnetism basics

An AC load has an active power of 800 W and an apparent power of 1000 VA. What is its reactive power? (Sinusoidal voltage and current.)

  1. A600 var
  2. B200 var
  3. C1800 var
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Answer: A. 600 var

Apparent power S, active power P and reactive power Q satisfy S² = P² + Q² (the power triangle). Q = √(1000² − 800²) = √360000 = 600 var, and the power factor is P/S = 0.8. Reactive power only shuttles back and forth between source and load and is not consumed on average, but it increases the current and therefore the losses in wiring and transformers.

  • A. Correct. √(1000² − 800²) = 600 var.
  • B. 200 var simply subtracts, ignoring the power triangle.
  • C. 1800 var simply adds the two.
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